For a hybrid vehicle, techniques of controlling engagement and disengagement of a clutch provided between the engine and the motor are described in the following references.
JP-A-2006-160104 discloses a control device in a hybrid four-wheel-drive vehicle including a clutch provided between the engine and the motor, front wheels driven by a traction force supplied from the engine and the motor, and rear wheels to which a traction force is supplied from another motor. The control device is configured to calculate a maximum traction force of the rear wheels on the basis of a predicted road friction coefficient and then to calculate a traction force of the front wheels from the demanded traction force and the maximum traction force of the rear wheels, so that it determines to engage the clutch in a case where the motor is not able to achieve the traction force of the front wheels. Also, the control device is configured to control the engine, the clutch, and the rear wheel motor by distributing the demanded traction force to the clutch according to the engagement determination when distributing the demanded traction force.
However, when the vehicle is driven while the clutch is disengaged, there is a difference of rotational frequencies between the clutch input and output shafts. When the clutch is engaged under these circumstances, the clutch is forced to slip, which raises a problem that the clutch durability is deteriorated and a shock reaches the vehicle.
JP-A-2003-237383 discloses a configuration provided with a clutch that is engaged and disengaged between the motor and the transmission for power transmission, and, as means for controlling the clutch engagement and disengagement, means for controlling the rotational frequency of the motor by defining the rotational frequency on the clutch transmission side as the target rotational frequency when switching the clutch to an engaged state and means for waiting until a difference of the rotational frequencies between the target rotational frequency and the rotational frequency of the clutch transmission side falls within a predetermined allowable range to switch the clutch to the engaged state.
However, in a case where the engine is also connected to the motor side, there is a problem that it takes a time to increase or decrease the rotational frequency due to inertia and energy is consumed unnecessarily.
JP-A-2000-255279 discloses a configuration adopted in a hybrid vehicle provided with a clutch that transmits engine rotations to a traction system equipped with a motor and a CVT that can change steplessly a reduction ratio of the traction system after the motor. According to this configuration, when the clutch is to be engaged in a drive state where the temperature of the clutch exceeds a preset temperature, a speed ratio of the CVT is regulated so that the rotational frequency on the clutch output side becomes equal to or lower than the preset rotational frequency and the engine and the traction system are engaged at or lower than the preset rotational frequency. A power source can be consequently switched from the motor to the engine by performing engagement operation rapidly at a high temperature without having to perform a slip operation that accelerates wear of the clutch.
The CVT speed ratio is made variable in order to prevent deterioration of the clutch durability. However, while the clutch is disengaged, the motor rotational frequency of the motor that is driving the vehicle varies with a change of the CVT speed ratio. This raises a problem that the driver has to drive the vehicle in a poor motor efficiency region and a problem that torque driving the vehicle shifts from the value demanded by the driver.
JP-A-2004-308913 describes the control of engagement and disengagement of the clutch in a case where a drive state using the motor alone shifts to hybrid drive. For example, in a case where the driver of a strong hybrid vehicle that can be driven by a traction motor alone up to a certain point wishes to rapidly accelerate the speed from a state where the vehicle is driven by a light load motor alone, the engine has to be started first in order to obtain the maximum output and the clutch is engaged after the engine start, and then the drive state is shifted to the hybrid drive.
In a case where there is a large difference between the engine rotational frequency and the rotational frequency of the CVT input shaft at the time of clutch engagement, a significant shock is induced, which raises a need to interpose a torque converter or the like for absorbing a shock. In addition, in a case where it is waited until the rotational frequencies coincide with each other to engage the clutch, a waiting time is extended. The driver therefore cannot obtain acceleration as he wishes, which raises a problem of a delayed response.    Patent Document 1: JP-A-2006-160104    Patent Document 2: JP-A-2003-237383    Patent Document 3: JP-A-2000-255279    Patent Document 4: JP-A-2004-308913